treewide: Adapt to GtkHeaderBar API changes

https://docs.gtk.org/gtk4/migrating-3to4.html#adapt-to-gtkheaderbar-and-gtkactionbar-api-changes

- `show-close-button` replaced with `show_title_buttons` wherever
  applicable, respecting new default value of `TRUE`
- `title` removed and managed by setting window title if needed
    - NOTE: I'm not 100% sure if the title logic here matches the GTK3
      version. I also just removed the SIP account widget titles
      altogether, since it's I couldn't figure out where the widget gets
      placed.

Part-of: <https://gitlab.gnome.org/GNOME/calls/-/merge_requests/714>

src/calls-account-overview.c

@@ -441,7 +441,7 @@ calls_account_overview_init (CallsAccountOverview *self)
                     G_CALLBACK (on_key_pressed),
                     self->account_window);
 
-  gtk_window_set_title (GTK_WINDOW (self), _("Account overview"));
+  gtk_window_set_title (GTK_WINDOW (self), _("VoIP Accounts"));
 }